1. Technical Field to which the Invention Pertains
The present invention relates to a cathode active material for use in a nonaqueous electrolyte secondary battery, a process for preparing the same, and a nonaqueous electrolyte secondary battery containing the cathode active material. More particularly the present invention relates to an improvement in a cathode active material for use in a nonaqueous electrolyte secondary battery.
2. Description of Related Art
A lithium ion secondary battery has such a feature that it has a higher energy density (a larger amount of work taken out of a charged battery per unit volume of the battery) than the conventional mainstream nickel-cadmium secondary battery and nickel-hydrogen secondary battery. In view of this, the development of lithium ion secondary batteries is in energetic progress in keeping with the recent needs of miniaturization and weight reduction of portable electronic equipment.
A lithium cobalt composite oxide represented by the formula: LiCoO.sub.2 is now used as the cathode active material in a commercially available lithium ion secondary battery. LiCoO.sub.2 is such an excellent active material that a secondary battery produced using the same has a large charge-and-discharge capacity and a stable charge-and-discharge cycle durability. However, cobalt as one starting material is small in the amount of mineral resources containing the same, and hence is expensive. In view of this, utilization of a lithium nickel composite oxide represented by the formula: LiNiO.sub.2, and lithium manganese composite oxides having a spinel structure and represented by the formula: LiMn.sub.2 O.sub.4 and the like as the cathode active material alternative to LiCoO.sub.2 is under study. Among them, lithium manganese composite oxides are low in the price of starting materials and are least in the load on the environment. Thus, they are strongly desired to be put into practical use.
However, a secondary battery produced using a lithium manganese composite oxide as the cathode active material is insufficient in charge-and-discharge cycle durability. In other words, it involves a disadvantage that the capacity of the battery is deteriorated when it is repeatedly charged and discharged. This phenomenon is believed to occur due to the following properties of the lithium manganese composite oxide, for example, as mentioned in the report of Gummow et al. [see R. J. Gummow, A. deKock, and M. M. Thackeray, Solid State Ionics, Vol. 69, p.p. 59-67 (1994)].
The first property mentioned is that a disproportionation reaction of the following formula occurs particularly in a discharge state, with the result that Mn.sup.2+ ions generated therefrom are dissolved out into an electrolytic solution. It is an irreversible reaction. Accordingly, as Mn.sup.2+ ions are dissolved out into the electrolytic solution, crystals of the lithium manganese composite oxide are deteriorated. EQU 2Mn.sup.3+ .fwdarw.Mn.sup.4+ +Mn.sup.2+
The second property mentioned is that a tetragonal strain, believed to be attributed to the Jahn-Teller effect, occurs in these crystals also in a discharge state. This also generally deteriorates the charge-and-discharge cycle durability of the secondary battery produced using the lithium manganese composite oxide.
The use of a spinel compound obtained by substituting part of manganese with other element(s) and represented by the formula: Li.sub.x M.sub.y Mn.sub.(2-y) O.sub.4 (wherein M stands for a metal element such as Ti, Ge, Fe, Co, Cr, Zn, Ni, or Al; x is a number exceeding 0 but not more than 1; and y is a number exceeding 0 but not more than 1) has been proposed in order to attain improvements in respect of these demerits of the lithium manganese composite oxide to thereby improve the charge-and-discharge cycle durability of the secondary battery produced using the same [e.g., see J. M. Tarascon, D. Guyomard, Electrochem. Soc., Vol. 139, p. 937 (1991)]. It has become apparent that the substitution of manganese with cobalt in particular among these metals is effective in suppressing deterioration of crystals of the lithium manganese composite oxide. In fact, secondary batteries produced using as the respective cathode active materials composite oxides respectively obtained by substituting part of manganese with these metal elements, including cobalt, were all found to be improved in charge-and-discharge cycle durability (in comparison with the initial secondary battery based on the lithium manganese composite oxide). In such batteries, however, a large decrease in initial charge-and-discharge capacity was observed [e.g., see Ri et al., page 181 of Preprints of 36th Denchi Toron-kai (36th Symposium on Batteries) held under the auspices of Denchi Gijutu Iinkai (Battery Technology Committee) of The Electrochemical Society of Japan (Sep. 12 to 14, 1995 in Kyoto).